The intelligence assessment, which had not previously been reported, seemed to counter the sentiments expressed by US President Donald Trump, who tweeted after his June 12 summit with Kim in Singapore that there was no longer a nuclear threat from North Korea, NBC said.
Kim had pledged to work toward the "complete denuclearisation" in return for security guarantees from the US.
"There's no evidence that they are decreasing stockpiles, or that they have stopped their production", NBC quoted one USA official as saying.
USA officials believe that North Korea is working to hide some of its nuclear stockpile and key aspects of its production facilities from the United States, The Washington Post reported Sunday.
"North Korea has covert facilities that it is unlikely to declare and eliminate initially", wrote the scientists. "Canadians, we're polite, we're reasonable, but we also will not be pushed around", he said in June.
"I am not going to comment on any reports, true, untrue, or partially true, about intelligence".
A satellite image provided by DigitalGlobe shows the Punggye-ri test site in North Korea.
Pompeo has already visited Pyongyang twice since April to meet with Kim - the first time when he was still director of the Central Intelligence Agency - and there are discussions about a possible third trip to North Korea late next week but such a visit has not yet been confirmed. That site is estimated to have produced fissile material for as many as a couple of dozen warheads.More news: Mexican fans celebrate with South Koreans after win over Germany
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"The expansion suggests that, despite hopes for denuclearization, Kim Jong Un is committed to increasing North Korea's stockpile of nuclear-armed missiles", the report's authors David Schmerler and Jeffrey Lewis wrote. It is believed to have twice the uranium enrichment capacity of the lone enrichment facility acknowledged by Pyongyang.
The recent reports, based on United States intelligence leaks, suggest the country is still upgrading its nuclear enrichment sites, among other activities. The Defense Intelligence Agency is at the high end with an estimate of about 50, but all the agencies believe Pyongyang is concealing an unknown number, especially smaller tactical ones, in caves and other underground facilities around the country. Some aspects of the new intelligence were reported Friday by NBC News.
Evidence collected since the June 12 summit in Singapore points to preparations to deceive the United States about the number of nuclear warheads in North Korea's arsenal as well as the existence of undisclosed facilities used to make fissile material for nuclear bombs, according to the report. "What our experts have devised is a program that with North Korean cooperation we can get physically - we would be able to dismantle the overwhelming bulk of their programs within a year".
Asked by senators on Wednesday about the status of private talks with North Korean officials, he declined to offer specifics.
Mr. Trump insisted the talks are moving "very quickly" and that he believed North Korea is serious about total denuclearization.
Pompeo told lawmakers on Wednesday that North Korea remains a nuclear threat, but defended Trump's earlier remarks that North Korea no longer remains a nuclear threat. "I don't think there's any doubt about that". "And it is to North Korea's advantage to dismantle very quickly".
The accuracy and completeness of the inventory, which the USA will able to partially verify based on spy satellite images and other intelligence, will tell whether North Korea is serious about giving up the nukes.